In practically operated electrical devices, due to the variations of manufacturing process, electric field design, improper type-selection, insulation material performance, assembling, etc., under the influences of fluctuating operating voltage, load current, consequent vibration and heat, etc., there are positions of electric field distortion or enhancement, which lead to partial discharge (PD).
Partial discharge can only be controlled but cannot be avoided, and its impacts on the performances of electrical devices are different due to different locations, shapes and types. Burrs on metal conductors will have their sharp corners ablated and rounded under prolonged discharging, and thus the degree of electric field distortion will weaken and the discharging phenomenon might disappear. Nevertheless, if the sharp corners get close to insulator surface, insulation may be deteriorated, causing risks that cannot be pre-controlled. Conductive particles in a fixed state are generally risk-controllable; nevertheless, in the case that the conductive particles have moving and vibrating characteristics, they would cause insulation failure under certain circumstances. Under the effect of the distorted electric field, discharging occurs continuously in air gaps inside the insulating material, which actually short-circuit the discharging air gap in the case that the inside of the discharging cavity is covered with carbonized conductive traces, and thus the electric field distortion is controlled and the discharging phenomenon disappears. There may be, however, a long-term discharge, resulting in deterioration (electrical tree) appearing inside the insulation material, which eventually develops into perforating defects, further leading to insulation failure. Moreover, the decomposition capability of discharging to oil or gas insulation medium would lead to deterioration or failure of the performance of such type of insulation medium and further cause equipment accidents.
For partial discharge detection, the pulse current detection approach based on coupling capacitors is earliest established, and is also complete in terms of standards and technologies thus being most widely used in practice. Being limited to situations that the operating voltage needs to be disconnected in such as researching, labs, commissioning, maintenance, etc., similar to the pulse current detection approach, there is a detection approach based on operating voltage, which is used for electrical devices that have equivalent detection impedance such as lightning arresters. This detection approach is a typical detection approach utilizing an electric connection, with a detection frequency band usually between hundreds of KHz to a few of MHz. Nevertheless, such detection approach is limited in number and the application effect is unknown.
Partial discharge detectors based on ultrasound are classified into two classes, i.e. piezoelectric sensors and non-contact sensors, but accompanied mainframe hardware systems are basically similar, which both perform feature extraction on the signals and draw spectra for time, phase and amplitude of the signals under the power supply frequency. Discharging causes vibration of the insulation medium and the partial discharge detection is performed by detecting the vibration signal after the vibration is transmitted to the distance. Such detection belongs to sound detection approaches, and the detection frequency band is usually within 1 MHz and is on the order of hundreds of KHz.
Any form of discharge would generates electromagnetic wave, which is mainly based on a discharge channel equivalent to a radiation antenna composed of a number of electric dipoles. Partial discharge belongs to the weak discharging type, with a generally narrow discharging channel and weak energy. From the perspective of electric dipoles, generally, the antenna characteristics of the discharging channel of the partial discharge is comparatively obvious and easy to understand. Partial Discharge detection based on UHF (ultra high frequency) is to use an UHF receiving antenna as a detection sensor, and the accompanied mainframe hardware system generally performs the processing with the discharging “cluster-like” signals taken as a unit, i.e., by performing feature extracting on the “cluster” signals and draw spectra for time, phase and amplitude of the signals under the power supply frequency. Partial Discharge detection based on UHF is essentially similar to pulse current PD detection approach and the ultrasound PD detection approach in terms of processing means.
In fact, the sensor is indispensable, no matter for which partial discharge detection approach described above. The detection signal (usually a “cluster-like” wave signal) based on the sensor is input into the PD detection mainframe, processed by both hardware and software, and presented at software side in the form of sequential detection signal. With the widening of the application horizon of the abovementioned PD detection means, nowadays there is wider optional scope with respect to detection devices corresponding to the same principle. Nevertheless, it is an indisputable fact that devices based on the same principle but produced by different manufacturers are inevitably varied in performance. It is to be seriously considered for performance evaluation of different types of devices based on the same detection principle, especially for the fact that: the developing trend for the intelligentization of state grids of the present world and the large-scale application of live-line monitoring and even on-line monitoring for various electrical parameters of the electrical devices lead to the extensive application of the PD detection.
An approach based on analog voltage signal injection is set forth based on the abovementioned facts, which can execute performance testing on different types of PD detector mainframes. By application of this testing approach, the evaluation about the pros and cons of the performances of various detection devices can be realized, providing essential technical support for the selection and network application in power grid of such PD detection devices.